The present invention relates to a process for obtaining sucrose from sugar cane.
The production of cane sugar for human consumption generally comprises two distinct operations, namely the production of raw sugar and the production of refined sugar. Production of raw sugar typically takes place at a sugar mill. In the mill, sugar cane stalks are chopped into pieces and the pieces are crushed in a series of mills in order to extract the juice. The juice from the first set of roller mills is referred to as xe2x80x9cfirst juice,xe2x80x9d while the total juice from all the roller mills in the process is referred to as xe2x80x9cmixed juice.xe2x80x9d The juice is normally limed, deaerated and clarified (i.e., removal of suspended solids, usually by sedimentation). The clarified stream is referred to as xe2x80x9cclarified juice.xe2x80x9d The juice is then evaporated to a thick syrup (known as xe2x80x9cevaporated juicexe2x80x9d or xe2x80x9cthick juicexe2x80x9d), and crystallized in a vacuum pan. The xe2x80x9cmassecuitexe2x80x9d (i.e., mixture of sugar syrup and crystals) produced in the vacuum pan is stirred in a crystallizer, and the mother syrup is spun off from the raw sugar crystals in a centrifugal separator. The solid sugar in the centrifugal basket is washed with water to remove remaining syrup. The solid crystalline product is termed xe2x80x9craw sugar.xe2x80x9d The syrup remaining after multiple stages of crystallization and centrifugation is referred to as xe2x80x9ccane mill molassesxe2x80x9d and is typically used for animal feed or fermentation syrups.
Raw sugar from the mill is usually transported to a sugar refinery for further processing. In a conventional cane sugar refining process, the raw sugar is first washed and centrifuged to remove adherent syrup, and the xe2x80x9caffined sugarxe2x80x9d thus produced is dissolved in water as xe2x80x9cmelter liquor.xe2x80x9d The syrup removed from the surface of the raw sugar is known as xe2x80x9caffination syrupxe2x80x9d and is broadly similar in composition to the mother syrup from the raw sugar crystallization. The affination syrup is processed in a xe2x80x9crecovery sectionxe2x80x9d through a series of vacuum pans, crystallizers and centrifugal separators similar to those used for the production of raw sugar, to recover an impure crystalline sugar product which has approximately the same composition as raw sugar. This recovered sugar product is dissolved in water, along with the affined raw sugar, to make melter liquor. The syrup remaining after the multiple stages of crystallization and centrifugation is referred to as xe2x80x9ccane refinery molasses,xe2x80x9d and is typically used for animal feed or fermentation syrups.
The melter liquor is purified, generally by the successive steps of clarification and decolorization, and the resulting xe2x80x9cfine liquorxe2x80x9d is crystallized to give refined sugar (also known as xe2x80x9cwhite sugarxe2x80x9d). The clarification step usually involves forming an inorganic precipitate in the liquor, and removing the precipitate and along with it insoluble and colloidal impurities which were present in the melter liquor. In one of the clarification processes commonly used for melter liquor, termed xe2x80x9ccarbonatationxe2x80x9d or xe2x80x9ccarbonation,xe2x80x9d the inorganic precipitate is calcium carbonate, normally formed by the addition of lime and carbon dioxide to the liquor. The calcium carbonate precipitate is usually removed from the liquor by filtration. Another clarification process, termed phosphatation, involves adding lime and phosphoric acid to the liquor, producing calcium phosphate precipitate.
The molasses produced in cane mills and refineries contains a substantial concentration of sucrose (e.g., 35-55% by weight on a dry solids basis). However, that sucrose cannot be recovered readily by additional crystallizations, because the molasses contains such a high concentration of impurities, including invert sugars (a mixture of glucose and fructose). The sucrose in the molasses could be sold for a far higher price than the molasses, if only the sucrose could be separated from the other constituents of the molasses in an economical way. However, the prior art has failed to provide a practical and cost-effective way to make this separation for cane syrups where invert is a significant component.
Chromatographic separation is used to desugar beet molasses and has been proposed for cane, but beet molasses has no invert and it is more straightforward to separate the sucrose. Chromatographic separation is an expensive process for cane.
Conventional dead end filtration is incapable of separating sucrose from macromolecular impurities in cane juice. Several methods of using microfiltration and ultrafiltration for purification of juice with reduced lime use have been reported, but these methods generally involve inserting microfiltration or ultrafiltration membranes into the conventional can process at one or more points.
There is a long-standing need for improved processes for obtaining sugar from cane that avoid or at least minimize one or more of the problems existing in the previously used processes.
The present invention relates to a process for producing sugar from cane. A sucrose-containing feed juice that has been obtained from sugar cane is filtered through a first ultrafiltration membrane that has a first molecular weight cutoff. This ultrafiltration step produces a first ultrafiltration permeate and a first ultrafiltration retentate. The first ultrafiltration permeate is filtered through a second ultrafiltration membrane that has a second molecular weight cutoff that is lower than the first molecular weight cutoff. This second ultrafiltration step produces a second ultrafiltration permeate and a second ultrafiltration retentate. The second ultrafiltration permeate is nanofiltered through a nanofiltration membrane, thereby producing a nanofiltration permeate and a nanofiltration retentate. The nanofiltration retentate has a higher concentration of sucrose on a dry solids basis than the feed juice introduced into the first ultrafiltration step, and can be used in evaporation and crystallization operations to produce crystals of white sugar.
In one embodiment of the invention, the sucrose-containing feed juice is manufactured by macerating sugar cane or pieces thereof, thereby producing a macerated material that comprises pulp and liquid, and then separating the liquid in the macerated material from the pulp, for example by one or more of centrifugation, conventional filtration, or screening. In one particular embodiment, the cane is macerated by first passing it through a hammer mill, and optionally it can subsequently be passed through a grinder, whereby the cane is converted into a mixture of pulp and sucrose-containing liquid. Preferably, no more than about 5% by weight of the sucrose present in the cane remains in the pulp after the liquid is separated therefrom, more preferably no more than about 3%.
After separation of the fibrous pulp from the liquid, and before the first ultrafiltration, the process can optionally include an additional step or steps to remove residual beet cane and silt from the separated liquid (juice). This can be done by screening and/or filtration. Preferably the screening or filtration removes at least 90% by weight of all fibers and silt having a largest dimension of about 150 xcexcm or greater, more preferably at least 90% by weight of all fibers and silt having a largest dimension of about 50 xcexcm or greater.
It is preferred to adjust the pH of the feed juice to about 6-8, for example by the addition of a base, prior to ultrafiltration. This can help minimize formation of invert.
The first ultrafiltration membrane preferably has a molecular weight cutoff between 2,000 daltons and a pore size no greater than about 0.2 microns. More preferably, it has a molecular eight cutoff of about 4,000-200,000 daltons. The first ultrafiltration permeate preferably has a color of about 3,000-15,000 icu. (All color values given herein are determined on an ICUMSA scale.)
The process of the present invention can be operated at a number of different process conditions. As representative examples of such conditions, the feed juice can be at a temperature of about 140-200xc2x0 F. during the first ultrafiltration, more preferably about 160-185xc2x0 F.
The second ultrafiltration membrane preferably has a molecular weight cutoff of about 500-5,000 daltons, more preferably about 1,000-4,000 daltons. In one particular embodiment of the process, the second ultrafiltration is performed in two stages, the first stage using an ultrafiltration membrane having a molecular weight cutoff of about 3,500-4,000 daltons, and the second stage using an ultrafiltration membrane having a molecular weight cutoff of less than about 3,500 daltons. The second ultrafiltration permeate preferably has a color no greater than about 8000 icu, more preferably no greater than about 4000 icu.
In order to minimize loss of sucrose in the retentate from the first and second ultrafiltration steps, it is preferable to include diafiltration steps in the process. xe2x80x9cDiafiltrationxe2x80x9d is used herein to mean ultrafiltration that employs added water in the feed to help flush sucrose through the membrane.
In one such embodiment of the process, the first ultrafiltration retentate is diafiltered through at least a first diafiltration/ultrafiltration membrane. This produces a first diafiltration permeate and a first diafiltration retentate. The first diafiltration permeate is then combined with the first ultrafiltration permeate and filtered through the second ultrafiltration membrane.
Similarly, the retentate from the second ultrafiltration can be diafiltered through at least a second diafiltration/ultrafiltration membrane. This second diafiltration step produces a second diafiltration permeate and a second diafiltration retentate. The second diafiltration permeate is then combined with the second ultrafiltration permeate and subsequently filtered through the nanofiltration membrane.
The retentates from the first and second ultrafiltrations (or diafiltrations) and the nanofiltration permeate can be combined to produce molasses. This combined stream may need to be concentrated by evaporation of water.
In addition to purification of the juice by nanofiltration, it is possible to include in the process ion exchange and/or electrodialysis purification steps. These three purification methods can be used in any sequence. In one particularly preferred embodiment of the process, the nanofiltration retentate is purified by electrodialysis, thereby producing a electrodialyzed juice and an electrodialysis residue, and then the electrodialyzed juice is purified by ion exchange, thereby producing a purified juice. Preferably, no lime and no carbon dioxide are contacted with any of the permeates.
The nanofiltration removes ash (including mono- and divalent cations), invert, organic acids, nitrogenous material and other low molecular weight organic or charged compounds. The nanofiltration and the optional electrodialysis and/or ion exchange preferably remove at least about 65% by weight of the Ca, Mg, K, Na and their associated inorganic and organic anions that are present in the second ultrafiltration permeate. The ion exchange replaces remaining divalent cations such as calcium and magnesium with monovalent cations such as potassium and sodium. Preferably, the nanofiltration retentate has a lower concentration of divalent cations on a dry solids basis than the second ultrafiltration permeate.
The nanofiltration permeate will contain a large percentage of the impurities that were present in the feed juice. For example, in many instances, the nanofiltration permeate will comprise at least about 30% by weight on a dry solids basis of the ash, and at least about 30% of the invert.
The purified juice (i.e., after nanofiltration and any electrodialysis and/or ion exchange), preferably has an ash concentration of no greater than about 2.5% by weight on a dry solids basis, more preferably no greater than about 2%, most preferably no greater than about 1.0%.
After the membrane filtration steps (and any electrodialysis and/or ion exchange), water can be evaporated from the purified juice to produce a concentrated syrup (e.g., 75% dry solids). White sugar can then be crystallized from the concentrated syrup. Because of the high degree of removal of impurities, the present invention can achieve two crystallizations of white sugar from the concentrated syrup.
A mother liquor will remain after one or more crystallizations of white sugar from the concentrated syrup. This mother liquor can be recycled to the second ultrafiltration. Optionally, this recycle stream can be further purified to reduce its ash and colour.
The process can optionally include sulfitation of one or more process streams. In particular, at least one aqueous stream selected from the group consisting of the feed juice, the first ultrafiltration permeate, the second ultrafiltration permeate, the nanofiltration retentate, and the evaporator feed can be contacted with an agent selected from the group consisting of sulfur dioxide, sulfite salts, bisulfite salts, metabisulfite salts, dithionites, and mixtures thereof, in an amount sufficient to provide an equivalent concentration of sulfur dioxide in the stream of at least about 100 ppm.
One particularly preferred embodiment of the invention is a process for producing sugar from cane that comprises the steps of:
(a) macerating sugar cane or pieces thereof, thereby forming pulp that comprises sucrose-containing aqueous liquid;
(b) separating the sucrose-containing liquid from the pulp;
(c) filtering the sucrose-containing liquid through a first ultrafiltration membrane that has a molecular weight cutoff of about 4,000-200,000 daltons, thereby producing a first ultrafiltration permeate that has a color no greater than about 15,000 icu and a first ultrafiltration retentate;
(d) filtering the first ultrafiltration permeate through a second ultrafiltration membrane that has a molecular weight cutoff of about 2,000-4,000 daltons, thereby producing a second ultrafiltration permeate that has a color no greater than about 8,0004,000 icu and a second ultrafiltration retentate;
(e) filtering the second ultrafiltration permeate through a nanofiltration membrane; thereby producing a nanofiltration permeate and a nanofiltration retentate, wherein the nanofiltration retentate has a higher concentration of sucrose on a dry solids basis than the sucrose-containing liquid in step (b);
(f) purifying the nanofiltration rententate by at least one method selected from the group consisting of ion exchange and electrodialysis, thereby producing an evaporator feed;
(g) evaporating water from the evaporator feed to produce a concentrated syrup; and
(h) crystallizing white sugar from the concentrated syrup.
Optionally, this embodiment of the process can further comprise the steps of:
(i) crystallising a mother liquor from the first crystallisation to produce white sugar;
(j) treating the mother liquor from the second crystallisation by chromatographic separation; and
(k) recycling the treated mother liquor back to the nanofiltration feed or the evaporator feed.
The various aspects of the present invention have a number of advantages over prior art cane processes. For example, the process of the present invention eliminates the need for producing raw sugar, and then having to redissolve or melt and refine this raw sugar. The present invention allows elimination of the carbonation process, and reduces the energy used because refining is eliminated.
The present invention provides a cost-effective way of reducing the ash content of the cane juice or syrup, preferably to about 2.5% or less (on a dry solids basis), more preferably to about 1.5% or less, most preferably to about 1% or less. This reduction in ash content is important because it allows a second strike of sucrose crystals from the syrup. In prior art cane processes, ash contents in the range of 6.0% made it practically impossible to have more than one strike of sucrose crystals.
In addition, the present invention can eliminate the need for desugarization of molasses streams. The efficient membrane filtration steps prevent excessive amounts of sucrose from entering the molasses streams in the first place.
Further, the present invention provides an economical and reliable method for removing color-causing materials from cane juice.